1. Field of the Invention
This invention relates to a method for producing (meth)acrylic acid performing a step of discarding part of the exhaust gas of an absorption column and recycling the remainder to a reactor and a step of separating a condensable substance exclusively from the recycling gas, and/or a method for producing (meth)acrylic acid comprising a step of decreasing the water content entrained in a molecular oxygen-containing gas supplied to the reactor, thereby enhancing an absorption efficiency of (meth)acrylic acid.
2. Description of the Related Art
Commercial production of acrylic acid generally resorts to the method of propylene oxidation which consists in subjecting propylene and/or acrolein to catalytic gas phase oxidation. When acrylic acid is produced by this method of propylene oxidation, the step of propylene oxidation gives rise to such impurities as water, acids like propionic acid, acetic acid, and maleic acid, and aldehydes like acrolein, furfural, and formaldehyde and ketoses like acetone, in the form of by-products. The gas containing these by-products is absorbed as an acrylic acid-containing solution generally via contact with an absorption solvent. This solution is subsequently purified by separating the absorption solvent by such a means as distillation and further separating low boiling substances and high boiling substances. The minute amount of such impurities as aldehydes which cannot be easily separated by distillation is possibly purified by a chemical treatment or a process of crystallization. The purification to a high degree necessitates many steps and complicates equipment and operation and forms one cause for degrading the yield of acrylic acid.
One known method, for example, produces acrylic acid of high purity by absorbing an acrylic acid-containing gas resulting from catalytic gas phase oxidation with a high boiling solvent, distilling the solvent now entraining the gas thereby separating it into the solvent and crude acrylic acid, and subsequently subjecting the crude acrylic acid to a process of crystallization (JP-A-1997-227445). This method, however, forms a complicated procedure which comprises a step of cooling an acrylic acid-containing gas with a venturi, then subjecting the cooled gas to a step of absorption and subsequently to a step of removing low boiling substances, thereafter a step of separating the residue in a distillation column into a high boiling substance and a medium boiling substance, and a step of extracting crude acrylic acid from the intermediate stage of the column.
If an acrylic acid solution having a high concentration is successfully treated at the step of acrylic acid production, this treatment will prove efficient in allowing a decrease in the amount of treatment at the subsequent purifying step. Thus, a method has been proposed which comprises supplying a reaction composition containing more than 7 vol % of propylene, molecular oxygen, steam, and the balance of an inert gas to a reactor having disposed therein numerous reaction tubes each packed with a catalyst and furnished with two reaction zones thereby allowing utility of a propylene reactant of a high concentration (JP-A-2000-103761). In one example 1 cited in this official gazette, the absorption with water obtained an acrylic acid solution having an average concentration of 69.5 wt %.
Another method has been disclosed which comprises introducing an acrylic acid-containing gas into the absorption column, introducing a recovery water containing acetic acid emanating from the bottom liquid of a solvent recovering column at the purifying step into the top of the absorption column thereby effecting absorption of acrylic acid therein, and producing as the bottom liquid of the absorption column an acrylic acid-containing solution composed of 50-80 wt % of acrylic acid, 2-5 wt % of acetic acid, and the balance of water (JP-A-1993-246941). This method obtains purified acrylic acid by subjecting the acrylic acid-containing solution to azeotropic dehydration using a mixed solution of two or more azeotropic solvents and subsequently passing the product of dehydration through such steps as the removal of high boiling substance.
Still another method has been disclosed which, in the absorption with water of an acrylic acid-containing gas resulting from a reaction of catalytic gas phase oxidation, comprises supplying the recovery water emanating from the step of azeotropic dehydration to the absorption column, supplying the resultant acrylic acid-containing solution to a stripping column, and obtaining an acrylic acid solution composed of 70.9 wt. % of acrylic acid, 25.6 wt. % of water, and 2.0 wt. % of acetic acid via the bottom of the stripping column (JP-A-2001-199931). This method obtains purified acrylic acid by performing azeotropic dehydration of the acrylic acid-containing solution and subsequently subjecting the product of dehydration to the step of crystallization.
The methods published in the official gazettes mentioned above, however, necessitate a stripping column for the purpose of obtaining an aqueous solution containing acrylic acid at a high concentration and, when an organic solvent is used as an absorption solvent, subsequently necessitate a step of solvent separation. Further, they have such a complicated procedure as adjusting anew the conditions of the reaction of catalytic gas phase oxidation. In spite of these measures, the acrylic acid-containing solutions they produce have concentrations which hardly deserve to be called fully satisfactory. In JP-A-1997-117445 mentioned above, for example, since the high boiling solvent used therein has a lower absorbing power than water, the acrylic acid concentration of the solution obtained at the step of absorption is about 20 wt. % at the most.
When the acrylic acid concentration is low in the acrylic acid-containing solution obtained from the step of absorption, this low concentration results in adding to the impurities to be separated at the subsequent steps and requiring the equipment for the separation to be enlarged and inevitably entailing an increase in the amount of necessary utilities. An attempt to heighten the acrylic acid concentration in the acrylic acid-containing solution obtained at the step of absorption is actually infeasible commercially because it increases the loss of acrylic acid at the step of absorption. In the inventions of JP-A-2000-103761, JP-A-1993-246941, and JP-A-2001-199931, the acrylic acid concentrations in the solutions obtained at the step of absorption are 80 wt. % at the most and the losses of acrylic acid, therefore, are presumed to be high.